Generally for recreational diving you probably can't tell the difference in performance between any top end reg combi.
I have the Mk25/S600 and the Mk17/S555. Used to have Apeks DS4/TX50 and even Mares MR12 as a first stage. Don't feel the difference.
the different will only kick in after certain depth.. heehee. my friend combi his FSR with another (dont know what brand) second stage complaining suck almost no air at depth.. lol
thought you have 2 regs? no?
well to describe my experience with mk25, it is like the air flows into my mouth from the mouthpiece (2nd stage) without having to suck air out of it.
Not to say that Apeks is inferior. There just is a difference from my own perspective and others who have loaned my reg.
it is about regulator tuning
the scubapro mk17/mk25 with S600 or A700 is about the same as Apeks FST/FSR or even DS4/DST. other than "feeling" different, i cant really tell whats the differences.
tested XTX200.. Black Pearl.. MK25/A700.. MK25/S600 at 57m. air share at this depth.. suck from my buddy's scubapro.. still cant tell the performance differences